The invention relates to a reversible thermal coloring composition that can be freely colored and decolored by thermal energy, a recording medium comprising said composition and a method for recording using said composition.
Thermal recording paper is widely used in a word processor and a facsimile machine. Nowadays, used thermal recording paper is not regenerated but always discarded on account of technological and economic limitations for regeneration.
Recently, thermal recording paper which can be reversibly used is desired on account of rapid increase of the amount of waste in a city and the need for preservation of environment and natural resource.
Several materials which can be thermally colored and decolored have been proposed. For example, JP-A-50-81157 and JP-A-50-105555 disclose materials comprising a leuco compound, a phenol compound and a higher alcohol. However, the color of the materials changes depending on temperatures and both of the colored and decolored states of the materials can not be fixed at ordinary temperature.
JP-A-60-193691 discloses a material wherein a leuco dye is combined with phloroglucinol. This material is colored by heat but a recording medium comprising the material is impractical since water or steam is used to decolor the colored material.
JP-A-61-237684 discloses a material wherein a leuco compound is combined with a compound such as phenolphthalein or thymolphthalein. Colored state of the material is obtained by heating and subsequent slow cooling of the material. The decolored state of the material is obtained by heating the colored material at a temperature higher than the coloring temperature of the material and subsequently rapidly cooling. However, an image having a high contrast cannot be obtained by use of a recording medium comprising the material since the material cannot be sufficiently decolored by heating and cooling.
JP-A-62-140881, JP-A-62-138568 and JP-A-62-138556 disclose a composition comprising a leuco compound, a phenol compound and a carboxylic acid ester. This composition maintains a colored state at a lower temperature and a decolored state at a higher temperature and is in a colored or decolored state at an intermediate temperature. However, a recording medium comprising this material is also impractical in that such recording medium must be kept at a specific temperature for preserving the image recorded thereon.
While, JP-A-2-188294, JP-A-2-188293, JP-A-4-46986, JP-A-4-50289, JP-A-4-50290, JP-A-5-177931, JP-A-5-92661 and JP-A-5-262032 disclose and a color-developing and decoloring agent having both color-developing and decoloring actions. Said color-developing and decoloring agent includes amphoteric compounds having acid group(s) and basic group(s) such as aminophenols, aminobenzoic acids and hydroxyaminobenzoic acids; or salts or complex salts formed from acidic substances such as gallic acid, bishydroxyphenylacetic acid and bishydroxyphenylbutyric acid and basic substances such as aliphatic and aromatic amines.
Particularly, JP-A-2-188294 discloses a composition comprising a leuco compound and the above salt or complex salt. These salts and complex salts are obtained by a reaction of said acidic substance with an equivalent amount of said basic substance.
Said amphoteric compounds and said salts or complex salts of the above prior arts have both the functions of a color-developer and a decoloring agent. Therefore, in these systems, both coloring and decoloring reactions proceed simultaneously in a molten and mixed state. However, the coloring reaction proceeds faster than the decoloring reaction. Consequently, said compositions are thought to be colored by heating for a short time, and to be decolored and equilibrated when being heated for a long time and kept molten. This decolored state is maintained after said compositions are cooled.
However, on account of the above coloring and decoloring processes, a recording medium comprising any of said compositions has a problem that it has a difficulty in controlling thermally its coloring and decoloring processes and a problem of reducing in color density with time. Further, said medium is colored by rapid cooling. Therefore, only a positive colored image in which the areas corresponding to an image such as a printed letter are colored can be obtained and a negative image in which the background is colored and the areas corresponding to an image such as a printed letter are decolored can be hardly formed on said recording medium, in a common image forming process wherein a thermal head is used.
The present inventors previously proposed a reversible coloring composition comprising a leuco compound and a developer having a long alkyl chain (JP-A-5-124360) to solve the problems of the above recording materials. A recording medium comprising this composition exhibits excellent coloring stability and decoloring properties. However, it has a problem that its decoloring is slightly slower than its coloring. In addition, this medium is colored by rapid cooling and only a positive image is formed on this medium but a negative image is hardly formed.
Additionally, a reversible recording medium in which a leuco compound, a developer which is an acidic substance, and a complex salt of an acid and an amine is disclosed with JP-A-6-48028. In this recording medium, a compound which is different from the developer is used as the acidic substance forming the complex salt with the amine. Like the above recording medium of JP-A-2-188294 using a complex salt, this recording medium is colored by short time-heating and rapid cooling, and is decolored by long time-heating and slow cooling or by heating to a lower temperature. Therefore, only a positive image can be formed but no negative image is obtained.
In the conventional reversible thermal coloring composition comprising a colorless leuco compound, an acidic substance and a basic substance, the lactone ring of the leuco compound is cleaved by the acidic substance of the composition and the leuco compound is brought into a colored state. The lactone ring of the colored leuco compound is again formed by the action of the basic substance of the composition and the leuco compound returns to colorless.
In short, the acidic substance acts as a developer of the leuco compound and the basic substance acts as a decoloring agent.
In the conventional thermal coloring composition as disclosed with JP-A-2-188294, wherein an amphoteric compound or a salt or complex salt of an acidic substance and a basic substance acting as both a developer and a decoloring agent is incorporated, both coloring and decoloring reactions proceed simultaneously when said composition is heated to melt and mix. However, said composition is thought to be colored when being heated for a short time since the coloring reaction proceeds faster than the decoloring reaction. While, said composition reaches to an equilibrium wherein said composition is in the decolored state when being heated for a long time and left melted. The decolored state is maintained even after being cooled.
The present inventors thought that among the above phenomena, not only the difference in the reaction rate between the coloring and decoloring reactions but also the colored state being to be fixed at room temperature are essential. In other words, the inventors thought that attention should be paid to its solid state, that is, the structure of arranged molecules and its change with the temperature at which the composition is heated.
The inventors have found that an entirely different phenomenon from the previously known ones occurs when an acidic substance having a specific structure is combined with a specific basic substance in a specific ratio. The inventors have further found that the most essential factor is that both the acidic and basic substances have a long chain structure which cause the formation of a structure of arranged molecules and the change thereof and that the acidic and basic substances are incorporated in a specific amount ratio.
The inventors have reached the present invention on the basis of the above findings.